Analog filters are fundamental blocks in analog and mixed-mode signal processing. In telecommunication receiver systems, for instance, an analog filter located in front of an Analog-to-Digital-Converter (ADC) may be employed to reduce out-of-band noise and to reject adjacent undesired communication channels or to select a desired communication channel. Several techniques have been employed to realize such analog filters to be embedded in a single-chip silicon-based transceiver systems. The main types of filters used are so-called sampled-data systems and continuous-time systems.
The most widely used type of sampled-data filters is the switched capacitor filter, wherein resistors are replaced by switched capacitors, the switching frequency of which determines the value of the “resistor”. These filters require precise performances of their active devices, for example a large unity-gain-bandwidth of an operational amplifier used. This leads, in general, to an increased power consumption and an increased complexity. On the other hand, these filters have the advantage that they guarantee a very accurate frequency response without the use of any tuning system, since their coefficients or properties are mainly determined by the ratio of homogeneous quantities, for example by the ratios of capacitors used.
In contrast, continuous-time filters have in general less stringent requirements for the performance of their active devices, i.e. operational amplifiers may operate with lower unity-gain-bandwidth. However, time-constants or other performance parameters of these filters are defined by uncorrelated components, for example by a product of a resistance and a capacitance (R·C) or a ratio of transconductance to capacitance (gm/C). Therefore, a tuning system is needed to align the frequency response of these filters, in particular to compensate for component variations (in particular variations of resistors and capacitors used) from nominal values due to technological spread, aging, temperature etc. and to align the filter frequency response to a desired target frequency response, for instance if the filter is used in a multistandard telecommunication device which needs different frequency responses for different standards.
Possible tuning techniques known in the art are described for example in V. Gopinathan, Y. P. Tsividis, K:-S. Tan, and R. K. Hester “Design Considerations for High-Frequency Continuous-Time Filters and Implementation of an Antialiasing Filter for Digital Video”, IEEE Journal of Solid State Circuits, December 1990, pp. 1368–1378, in Y. Tsividis, “Self-tuned filters,” Electron. Lett., vol. 17, pp. 406–407, June 1981, in A. M. Durham and W. Redhman-Whhe, “Integrated continuous-time balanced filters for 16-b DSP interfaces,” IEEE J. Solid-State Circuits, vol. 28, pp. 835–839, July 1993, in J. B. Hugjes, N. C. Bird, R. S. Soin, “A Novel Digitally Self-Tuned Continuous-Time filter Technique”, Intern. Symp. On Circuits and Systems, ISCAS 1986, pp. 1177–1180, or in Y. P. Tsividis, “Integrated Continuous-Time Filter Design—An Overview”, IEEE J. Solid-State Circuits, March 1994, pp. 166–176. These techniques use different kinds of input signal patterns to be applied to the filter and evaluate the filter response to control or tune one or more parameters of the filter like gain, pole frequency or even the complete frequency response behavior.
For example, in pure analog systems, a signal with a fixed frequency or DC voltage is used as the input signal pattern. The use of an input signal with a single frequency results in a measurement of the effective filter frequency response to this frequency. However, in this case, noise can affect the tuning accuracy. Furthermore, for digital evaluation, a high resolution ADC is needed to sample the response of the filter. The use of a direct current (DC) voltage as an input signal does not include the measurement of a frequency response at all.
On the other hand, with increasing miniaturization of electronic circuits the possibility for downscaling a complete filtering system is an important issue.
It is therefore an object of the present invention to provide a method and an apparatus for tuning a filter which is easy to implement, downscalable and which gives the possibility to tune all desired filter parameters and takes the frequency response of the filter into account.